Low rf interference switching amplifier and method

ABSTRACT

A switching amplifier includes first and second output terminals that may be connected to a load. A pulse-width modulator receiving an input signal to obtain respective positive and negative values of the input signal. The modulator is connected to first and second switching circuits. The first switching circuit applies a plurality of pulses to the first output terminal that, in response to the positive samples, have a constant frequency and are pulse-width modulated, and, in response to the negative samples, have a varying frequency and a constant width. Similarly, the second switching circuit applies a plurality of pulses to the second output terminal that, in response to the negative samples, have a constant frequency and are pulse-width modulated, and, in response to the positive samples, have a varying frequency and a constant width. The varying phase of the constant width pulses disperses RF interference across a wider spectrum.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Application No.60/887,394, filed Jan. 31, 2007. The entire disclosure of the priorapplication is considered to be part of the disclosure of the instantapplication and is hereby incorporated by reference therein.

TECHNICAL FIELD

This invention relates to switching amplifiers and methods, and, moreparticularly, to a system and method for reducing the electromagneticinterference of switching amplifiers.

BACKGROUND OF THE INVENTION

Switching amplifiers provide far greater efficiency than their analogcounterparts, primarily because transistors used to switch voltages to aload are either turned ON, so that the voltage across the transistor isrelatively low, or turned OFF, so that the current through thetransistor is relatively low. With either a low voltage across thetransistor or a low current through the transistor, the power dissipatedby the transistor is relatively low.

Although conventional switching amplifiers are widely used, they canproduce excessive distortion in their output signals because ofcapacitive coupling between the gates of respective switchingtransistors used by such amplifiers. Another limitation of conventionalswitching amplifiers is that they can sometimes generate excessiveelectromagnetic radio frequency (“RF”) interference, that can interferewith the operation of the amplifier as well as with other electronicdevices in the vicinity of the amplifier. This RF interference can beattenuated to some extent by coupling the load driven by the amplifierto filters formed by inductors and/or capacitors. However, the remainingRF interference can still be too high in some applications.

Attempts have been made to minimize the distortion of signals outputfrom switching amplifiers by operating them in a balanced manner so thatcapacitive coupling to one side of a load is matched by capacitivecoupling to the other side of the load. While this approach issuccessful in minimizing signal distortion, it actually increases the RFinterference generated by the amplifier since the number of transistorsswitching must be increased.

Attempts have been made to minimize the frequency at which the peakamplitude of the RF interference occurs by varying to “dithering” theswitching times of the transistors, but doing so tends to increase thedistortion of the output signal since a signal input to the amplifier isnot sampled at regular intervals. In addition, spread spectrum EMIreduction is limited by audible distortion products at higherdeviations. This distortion is normally greater at higher outputamplitudes, so it clearly is a distortion since it increases withamplitude.

There is therefore a need for a system and method for operatingswitching amplifiers in a manner that minimizes the magnitude of RFinterference generated by the amplifier, and does so without introducingsignificant distortion.

SUMMARY

A switching amplifier and method includes first and second outputterminals to which a load may be connected. An input signal is appliedto switching amplifier. The switching amplifier applies a plurality ofperiodic first pulses to the first output terminal and it adjusts thewidths of the first pulses as a function of the magnitude of the inputsignal. The switching amplifier also applies a plurality of periodicsecond pulses to the second output terminal. The second pulses areasynchronous with, but substantially equal in number to, the firstpulses. If the input signal has positive and negative polarities, theswitching amplifier may obtain positive and negative samples,respectively. In response to the positive samples, the first pulses areperiodic and their widths are modulated, and the second pulses have aconstant width. In response to the negative samples, the second pulsesare periodic and their widths are modulated, and the first pulses have aconstant width. The widths of the modulated pulses may be greater thanthe width of the unmodulated pulses by a magnitude corresponding to theamplitudes of the input signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a prior art switching amplifier that can beoperated according to embodiments of the invention.

FIG. 2 is a timing diagram showing an example of how the switchingamplifier of FIG. 1 has been operated in the prior art.

FIG. 3 is a timing diagram showing an example of how the switchingamplifier of FIG. 1 has been operated in the prior art in an attempt tominimize output signal distortion.

FIG. 4 is a timing diagram showing an example of how the switchingamplifier of FIG. 1 has been operated in the prior art in an attempt tominimize electromagnetic RF interference.

FIG. 5 is a timing diagram showing the operation of the switchingamplifier of FIG. 1 according to one embodiment of the invention.

DETAILED DESCRIPTION

A typical prior art switching amplifier 100 is shown in FIG. 1. Theswitching amplifier 100 includes a pulse-width modulation (“PWM”)modulator 102 that receives a signal through an input line 101. The PWMmodulator 102 generates four outputs A-D, which drive the gates ofrespective transistors 103, 104, 105, 106, which are arranged in abridge or H configuration. The transistors 103, 104 are connected inseries between a supply voltage V+ and ground, and the transistors 105,106 are similarly connected in series between V+ and ground. A load 114is connected between a first output node E formed by the junctionbetween the transistors 103, 104 and a second output node F formed bythe junction between the transistors 105, 106.

As shown in FIG. 1, the load 114 is formed by a resistive load 110connected in parallel with a capacitor 109, both of which are connectedbetween a pair of inductors 107, 108. As is well known in the art, theinductors 107, 108 and capacitor 109 form an LC filter that reduces theamount of electromagnetic energy generated by switching the transistors103-106 ON and OFF. Although FIG. 1 shows the load 114 formed by aresistive load 110 and LC filter, the load 114 may instead be inductive,such as a MRI coil, capacitive, such as a piezoelectric acoustictransducer, or some combination on these impedance elements. Also,although transistors 103-106 are shown as being the devices used toswitch various voltages to the load 114 at various times, it will beunderstood that other switching devices may be used. Finally,configurations of switching devices other than that shown in FIG. 1 mayalso be used to apply various voltages to the load 114 at various times.

The operation of the amplifier 100 is shown in FIG. 2 in which sixwaveforms A-F have been labeled to show an example of the signalspresent at the corresponding nodes A-F when operating the amplifier 100in a conventional manner. The signal A is initially driven high to turnON the transistor 103 for a period corresponding to the amplitude of thesignal applied to the PWM modulator 102. At the same time and for thesame period, the signal B is driven low to turn OFF the transistor 104.The transistor 106 is ON during this time. As a result, the voltage V+is connected to ground through the inductor 108, resistive load 110 andinductor 107, thereby causing current to start flowing through theresistive load 110. Were it not for the filter formed by the inductors107, 108 and capacitor 109, the sudden increase in current through thetransistors 103, 106 resulting from turning the transistor 103 ON wouldresult in substantial RF interference. However, this RF interference isattenuated to some extent by the filter formed by the inductors 107, 108and capacitor 109.

At the end of the period corresponding to the amplitude of the signalapplied to the PWM modulator 102, the signal A transitions low to turnOFF the transistor 103, and the signal B transitions high to turn ON thetransistor 104. Again, if the inductors 107, 108 and capacitor 109 werenot present, substantial RF interference might be generated by switchingthe transistor 103 OFF.

A predetermined period later, the above sequence is repeated except thatthe transistor 103 is turned ON and the transistor 104 is turned OFF fora period that is shorter than the period that the transistor 103 waspreviously turned ON because the amplitude of the signal applied to PWMmodulator 102 is lower. As a result, a current flows through theresistive load 110 for a shorter period. The above sequence is thenrepeated again for an even shorter period corresponding to the loweramplitude of the signal applied to the PWM modulator 102.

Next, the polarity of the signal applied to the resistive load 110 isreversed by repeating the above sequence except that the transistor 105is turned ON instead of the transistor 103, and the transistor 106 isturned OFF instead of the transistor 104. Thus, when the input signal tothe PWM modulator 102 is positive, the pulse width of the signalsapplied to the transistors 103, 104 are modulated, and when the inputsignal to the PWM modulator 102 is negative, the pulse width of thesignals applied to the transistors 105, 106 are modulated.

The switching amplifier 100 operating as described above providesadequate performance in may cases, but it results in distortion that canbe excessive when low distortion amplification is required. The reasonfor this distortion is essentially the capacitive coupling between thegates of the transistors 103-106 and the nodes E, F, which effectivelydistorts the width of the pulses generated at the nodes E, F. When thiscapacitive coupling occurs, the width of these pulses no longercorrespond to the amplitude of the signal applied to the PWM modulator102.

One approach to reducing the distortion of the amplifier 100 operatingas described above is to operate the amplifier 100 as shown in FIG. 3 inwhich the same signal is applied to the PWM modulator 102 as in theexample shown in FIG. 2. As shown in FIG. 3, when the input signal tothe PWM modulator 102 is positive, the pulse width of the signalsapplied to the transistors 103, 104 are still modulated, and when theinput signal to the PWM modulator 102 is negative, the pulse width ofthe signals applied to the transistors 105, 106 are still modulated.However, when applying pulsewidth modulated signals to a first side ofthe load 114, unmodulated signals are applied to the first side of theload as well as second side of the load 114. Applying unmodulatedsignals to both sides of the load, mitigates differential chargeinjection. The transistors 103, 104 are switched ON and OFF respectivelyfor a period that is constant when the input signal to the PWM modulator102 is positive, and the transistors 105, 106 are switched ON and OFF,respectively, for a period that is constant when the input signal to thePWM modulator 102 is negative. When both transistors 103, 105 are turnedON, the effect is as if neither transistor 103, 105 was turned ON.However, the capacitive coupling from the gate of the transistor 103 tothe node E is matched by the capacitive coupling from the gate of thetransistor 105 to the node F. The periods during which the transistor103 is turned ON while the transistor 105 is OFF are identical to theperiods during which the transistor 103 is turned ON in the example ofFIG. 2, as can be seen by the signal E′-F′. As a result, the currentsthrough the resistive load when operating as in FIG. 3 are the same andof the same duration as when operating as shown in FIG. 2. Moreover, thecapacitive coupling of the signal A to the node E occurs the same numberof times that the signal C is capacitively coupled to the node F.Similarly, the capacitive coupling of the signal B to the node E occursthe same number of times that the signal D is capacitively coupled tothe node F. As a result, the capacitive coupling to the node E cancelsout the capacitive coupling to the node F, thereby preserving the widthof the pulses generated at the nodes E, F. Therefore, operating theamplifier 100 as shown in FIG. 3 results in very little distortion.

An additional problem with operating the amplifier 100 as shown in FIG.2 is the electromagnetic RF interference resulting from switching thetransistors 103-108 as explained therein. Operating the amplifier 100 asshown in FIG. 3 does not solve this problem. To the contrary, operatingthe amplifier 100 as shown in FIG. 3 can actually exacerbate the problemof RF interference because both transistors 103, 105 switch at the sametime compared to the operation as shown in FIG. 2 in which only one ofthe transistors, 103 or 105, switch. Similarly, in FIG. 3, bothtransistors 104, 106 switch at the same time. Since RF interference isgenerated each time one of the transistors 103-106 is switched, doublingthe number of transistors being switched increases the magnitude of theRF interference.

One approach to reducing the magnitude of the RF interference is to varyor “dither” the timing (but not the duration) at which the transistors103, 105 are turned ON as shown in FIG. 4 in which the signal applied tothe PWM modulator 102 is shown in the top line, the signal at the node Eis shown in the next line, and the signal at the node F is shown in thebottom line. As shown therein, the operation of the circuit is similarto the operation shown in FIG. 3. The unmodulated pulses again have aconstant width, and they again are provided to compensate for thecapacitive coupling through the transistors 103, 105 receiving thepulse-width modulated signals from the PWM modulator 102. The operationof the switching amplifier 100 shown in FIG. 4 differs from theoperation shown in FIG. 3 in that the time between switching thetransistors 103, 105 is not constant. Instead, for example, the durationof the period between the transistor 103 first being turned ON and thetransistor 103 being turned ON a second time is different from theperiod between with transistor 103 being turned ON a second time andthen turning ON a third time. By varying the conductive times of thetransistors 103-106 in this manner, the frequency at which the peakamplitude of the RF interference spectrum occurs is varied frompulse-to-pulse, thereby distributing the RF interference over a range offrequencies. In contrast, the frequency at which the peak amplitude ofthe RF interference spectrum occurs when operating as shown in FIG. 3 isthe same from pulse-to-pulse. As a result, the peak amplitude of the RFinterference at output sample rate is significantly higher whenoperating the amplifier 100 as shown in FIG. 3 compared to operating asshown in FIG. 4.

Although operating the amplifier 100 as shown in FIG. 4 significantlyreduces the magnitude of the RF interference, the amplifier 100 cannevertheless generate RF interference that can be excessive in someinstances. Additionally, operating the amplifier 100 as shown in FIG. 4can produce excessive distortion because the signal applied to the PWMmodulator 102 is not sampled at a constant rate. The technique disclosedin this embodiment causes low level constant distortion, such as tapehiss, which is not perceived to be distortion by most listeners.Dithering can therefore be pushed higher (reducing measured EMI) beforeencountering customer complaints.

According to one embodiment of the invention, the switching amplifier isoperated as shown in FIG. 5. The signal applied to the PWM modulator 102is again shown in the top line, the signal at the node E is shown in thenext line, and the signal at the node F is shown in the bottom line.When the input signal is positive, the width of the pulses at node E areagain modulated, and when the input signal is negative, the width of thepulses at node F are again modulated. The unmodulated pulses continue tohave a constant width, and they again are provided to compensate for thecapacitive coupling through the transistors 103, 105 receiving thepulse-width modulated signals from the PWM modulator 102.

The operation of the switching amplifier 100 according to one embodimentof the invention as shown in FIG. 5 differs from the operation shown inFIG. 4 by varying or “dithering” the times between only the unmodulatedpulses. The times between the PWM modulated pulses are constant. As aresult, the sampling times of the input signal can be constant, thusavoiding distortion in the output signal from the amplifier 100. Yet, byvarying or “dithering” the switching times of the unmodulated pulsesused to compensate for the capacitive coupling of the modulated pulses,the frequency of the peak amplitude of the RF interference is varied,thus minimizing the peak amplitude of the RF interference. Operating theamplifier 100 as shown in FIG. 5 thus produces relatively little RFinterference in a manner that does not result in output signaldistortion. The dithering of the unmodulated pulse switching times mayvary in a pseudo-random manner.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1. A method of operating a switching amplifier having an input terminaland a pair of first and second output terminals, the method comprising:applying an input signal to the input terminal during a first period oftime; applying a plurality of first pulses to the first output terminalof the switching amplifier during the first period of time; applying aplurality of second pulses to the second output terminal of theswitching amplifier during the first period of time, the number ofsecond pulses being substantially equal to the number of first pulses,the interval between the second pulses being different from the intervalbetween the first pulses; and pulse-width modulating the first pulsesduring the first period of time so that the widths of the first pulsesare determined by the input signal.
 2. The method of claim 1 wherein theinput signal alternates between positive and negative values, andwherein the first period of time comprises a period of time when theinput signal is positive, and wherein the method further comprises:applying an input signal to the input terminal during a second period oftime occurring when the input signal is negative; applying a pluralityof third pulses to the second output terminal of the switching amplifierduring the second period of time; applying a plurality of fourth pulsesto the first output terminal of the switching amplifier during thesecond period of time, the number of fourth pulses being substantiallyequal to the number of third pulses, the intervals between the fourthpulses being different from the intervals of the third pulses; andpulse-width modulating the third pulses during the second period of timeso that the widths of the third pulses are determined by the inputsignal.
 3. The method of claim 1 wherein the intervals between thesecond pulses being different from the intervals of the first pulsescomprises varying the intervals in the second pulses in a pseudo-randommanner.
 4. The method of claim 1 wherein the intervals between thesecond pulses being different from the intervals of the first pulsescomprises varying the intervals in the second pulses according to apredetermined algorithm.
 5. The method of claim 1 wherein the act ofpulse-width modulating the first pulses so that the widths of the firstpulses are determined by the input signal comprises pulse-widthmodulating the first pulses so that the widths of the first pulses aregreater than the widths of the second pulses by a value that isdetermined by the input signal.
 6. The method of claim 1 wherein theintervals between the second pulses being different from the intervalsof the first pulses comprises varying the intervals of the second pulsesin a non-periodic manner.
 7. A method of applying a signal to a loadhaving first and second terminals, the method comprising: applying aplurality of periodic first pulses to the first terminal of the load;adjusting the widths of the first pulses as a function of the magnitudean input signal; and applying a plurality of second pulses to the secondterminal of the load, the second pulses being asynchronous with, butsubstantially equal in number to, the first pulses.
 8. The method ofclaim 7, further comprising filtering the first and second pulsesapplied to the load to attenuate frequency of the first and secondpulses.
 9. The method of claim 7 wherein the widths of the first pulsesare adjusted only when the input signal has a positive polarity.
 10. Themethod of claim 7 wherein the intervals between the second pulses varyin a pseudo-random manner.
 11. The method of claim 7 wherein the act ofadjusting the widths of the first pulses as a function of the magnitudeof the input signal comprises adjusting the widths of each of the firstpulses so that they are greater than the widths of corresponding ones ofthe second pulses by respective amounts that are determined by themagnitude of the input signal.
 12. The method of claim 7 wherein theload comprises a resistive load.
 13. A method of applying a signal to aload having first and second terminals, the method comprising: samplingan input signal having positive and negative values to provide aplurality of positive and negative samples, respectively; in response toeach of the positive samples, applying a pulse to the first terminal ofthe load having a width that is a function of the magnitude of therespective positive sample, the pulses applied to the first terminal ofthe load responsive to the positive samples occurring at regularintervals; in response to each of the positive samples, applying a pulseto the second terminal of the load having a constant width, the pulsesapplied to the second terminal of the load responsive to the positivesamples occurring at irregular intervals; in response to each of thenegative samples, applying a pulse to the second terminal of the loadhaving a width that is a function of the magnitude of the respectivenegative sample, the pulses applied to the second terminal of the loadresponsive to the negative samples occurring at regular intervals; andin response to each of the negative samples, applying a pulse to thefirst terminal of the load having a constant width, the pulses appliedto the first terminal of the load responsive to the negative samplesoccurring at irregular intervals.
 14. The method of claim 13 wherein thepulses applied to the second terminal of the load responsive to thepositive samples and the pulses applied to the first terminal of theload responsive to the negative samples occur at random intervals. 15.The method of claim 13 wherein the act of applying a pulse to the firstterminal of the load in response to each of the positive samplescomprises applying a pulse to the first terminal of the load having awidth that is greater than the width of the pulses applied to the secondterminal of the load responsive to the positive samples by a magnitudecorresponding to the magnitude of the respective positive sample, andwherein the act of applying a pulse to the second terminal of the loadin response to each of the negative samples comprises applying a pulseto the second terminal of the load having a width that is greater thanthe width of the pulses applied to the first terminal of the loadresponsive to the negative samples by a magnitude corresponding to themagnitude of the respective negative sample.
 16. The method of claim 13,further comprising attenuating relatively high frequency components ofthe pulses applied to the first and second terminals.
 17. A switchingamplifier, comprising: a first switch having a control terminal, a firstterminal connected to a first supply voltage and a second terminalconnected to a first output node, the first switch being controlled by afirst control signal applied to the control terminal of the firstswitch; a second switch having a control terminal, a first terminalconnected to a second supply voltage and a second terminal connected tothe first output node, the second switch being controlled by a controlsignal applied to the control terminal of the second switch; a thirdswitch having a control terminal, a first terminal connected to thefirst supply voltage and a second terminal connected to a second outputnode, the third switch being controlled by a third control signalapplied to the control terminal of the third switch; a fourth switchhaving a control terminal, a first terminal connected to the secondsupply voltage and a second terminal connected to the second outputnode, the fourth switch being controlled by a control signal applied tothe control terminal of the fourth switch; and a modulator having aninput terminal coupled to receive an input signal and respective outputterminals coupled to the control terminals of the first, second, thirdand fourth switches, the modulator being operable to receive the inputsignal and, in response to the input signal, applying a pulse to therespective control terminals of the first and second switches to renderthe first switch conductive and the second switch non-conductive, thepulse applied to the respective control terminals of the first andsecond switches having a width that is determined by the input signal,the pulses applied to the respective control terminals of the first andsecond switches occurring at regular intervals, the modulator furtherbeing operable in response to the input signal to apply a pulse to therespective control terminals of the third and fourth switches to renderthe third switch conductive and the fourth switch non-conductive, thepulses applied to the respective control terminals of the third andfourth switches having a constant width and occurring at irregularintervals.
 18. The switching amplifier of claim 17 wherein the first,second, third and fourth switches comprises respective transistors. 19.The switching amplifier of claim 17, further comprising a filter coupledto at least one of the first and second output nodes.
 20. The switchingamplifier of claim 19 wherein the filter comprises: a first inductorhaving a first terminal and a second terminal connected to the firstoutput node; a second inductor having a first terminal and a secondterminal connected to the second output node; and a capacitor coupledbetween the first terminal of the first inductor and the first terminalof the second inductor.
 21. The switching amplifier of claim 17 whereinthe input signal may have positive and negative polarities, and whereinthe pulses applied to the control terminals of the first and secondswitches responsive to the positive input signal have respective widthsthat are a function of the magnitude of the input signal, and the pulsesapplied to the control terminals of the first and second switchesresponsive to the negative input signal have a constant width.
 22. Theswitching amplifier of claim 21 wherein the pulses applied to thecontrol terminals of the third and fourth switches responsive to thenegative samples have respective widths that are a function of the inputsignal, and the pulses applied to the control terminals of the third andfourth switches responsive to the positive input signal have a constantwidth.
 23. The switching amplifier of claim 17 wherein each of thepulses applied to the respective control terminals of the first andsecond switches have respective widths that are greater than the widthof the pulses applied to the respective control terminals of the thirdand fourth switches by an amount corresponding to the magnitude of theinput signal.
 24. A switching amplifier having first and second outputterminals, comprising: a circuit operable to receive and obtainrespective positive and negative values of an input signal applied tothe switching amplifier having positive and negative polarities; a firstswitching circuit coupled to the sampling circuit and to the firstoutput terminal, the first switching circuit being operable to apply aplurality of pulses to the first output terminal, the respective pulsesapplied to the first output terminal in response to the positive sampleshaving a constant frequency and being pulse-width modulated according tothe magnitudes of the respective positive samples, and the respectivepulses applied to the first output terminal in response to the negativevalues of the input signal having a varying frequency and a constantwidth; and a second switching circuit coupled to the sampling circuitand to the second output terminal, the second switching circuit beingoperable to apply a plurality of pulses to the second output terminal,the respective pulses applied to the second output terminal in responseto the negative samples having a constant frequency and beingpulse-width modulated according to the magnitudes of the respectivenegative values of the input signal, and the respective pulses appliedto the second output terminal in response to the positive values of theinput signal having a varying frequency and a constant width.
 25. Theswitching amplifier of claim 24 wherein, in response to each of thesamples, the switching circuits are operable to make the pulse-widthmodulated pulses have a width that is greater than the width of theconstant width pulses by an amount corresponding to the amplitude of therespective values of the input signal.